diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index e061622f9412..8b1c2febb889 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -1512,6 +1512,7 @@ template <typename F> class glibc_file : public file_base<F> {
   void init_buffer() {
     if (this->file_->_IO_write_ptr) return;
     // Force buffer initialization by placing and removing a char in a buffer.
+    assume(this->file_->_IO_write_ptr >= this->file_->_IO_write_end);
     putc_unlocked(0, this->file_);
     --this->file_->_IO_write_ptr;
   }